The problem with bubbles of received wisdom and herd-euphoria is conditions change but the risk of something untoward happening is still perceived as inconsequentially low.
All bubbles pop--and not just stock market bubbles. A speculative bubble is a psychological-social phenomenon in which confidence in the stability of future gains reaches levels where doubts are banished and risks have dissipated into thin air. This confidence can be euphoric or it can be the baseline: this (guaranteed gains) is the way the world works.